Fuel cells cause a fuel gas such as hydrogen and an oxidant gas such as air to electrochemically react at a gas diffusion electrode, thereby concurrently supplying electricity and heat. Such fuel cells are classified into several types according to the kind of the electrolyte used therefor. The polymer electrolyte used herein generally comprises a skeleton of —CF2— as its main chain and a sulfonic acid is attached to the terminal of its side chain.
A polymer electrolyte fuel cell is fabricated in the following manner. First, a paste for catalyst layer, prepared by mixing a dispersion of the above-described polymer electrolyte with a carbon powder carrying a platinum-based metal catalyst, is applied onto both surfaces of a membrane of this polymer electrolyte, and the whole was dried to form a catalyst layer, which will constitute electrodes (a cathode as an air electrode and an anode as a fuel electrode). On the outer surface of the catalyst layer, a porous conductive substrate such as a carbon paper is disposed as a gas diffusion layer, which will constitute the electrodes, for diffusing air and a fuel gas. In other words, the catalyst layer and the gas diffusion layer constitute the electrodes. Alternatively, the paste for catalyst layer may be applied onto the carbon paper constituting the gas diffusion layer, and the polymer electrolyte membrane may be bonded to this. This yields an electrolyte membrane-electrode assembly (MEA) comprising the polymer electrolyte membrane, catalyst layer and gas diffusion layer.
Conductive separator plates for mechanically fixing the MEA and electrically connecting adjacent MEAs in series are disposed on the outer surfaces of the MEA. The MEA and the separator plates are laminated to obtain a unit cell. A gas flow path for supplying a reactant gas (oxidant gas or fuel gas) to the electrode and transferring water produced by the reaction of hydrogen and oxygen, residual gas and the like, is formed on the separator plate. A carbon material having electrical conductivity, gas tightness and corrosion resistance is often used for the separator plate. However, because of the excellent moldability and cost-effectiveness as well as ease of thinning the separator, separators using a metal material such as stainless steel are also being investigated. Further, a sealing member such as a gasket or a sealing agent is arranged on the peripheries of the gas flow path, the electrode and the like to prevent the reactant gases from directly mixing or from leaking outside.
When the above-described unit cell is used as a power generating device, it is common to laminate a plurality of the unit cells in order to increase the output voltage. To the gas flow paths disposed on the separator plate, the fuel gas such as hydrogen and the oxidant gas such as air are supplied from outside through manifolds, and these gases are supplied to the gas diffusion layers of the respective electrodes. Current generated by the reaction of these gases at the catalyst layers is collected at the electrodes and is taken outside through the separator plates.
Herein, since the above-described polymer electrolyte exhibits hydrogen ion conductivity when it contains water, the fuel gas to be supplied to the fuel cell is generally humidified. In addition, since the cell reaction produces water at the cathode, water is always present within the cell. As a result, there is the possibility that ionic impurities, inorganic impurities and organic impurities contained in a carbon material, sealing material, resin material and metal material, each of which is the component of the cell, are eluted if the cell is operated for a long period of time.
Moreover, since air to be applied to the fuel cell contains, for example, trace amounts of air pollutants such as nitrogen oxides or sulfur oxides, the fuel gas is occasionally contaminated with traces amount of metal contained in the hydrogen generating device.
Further, such impurities are accumulated in the polymer electrolyte membrane, the catalyst layer at the electrode and the like, leading to a reduction in the conductivity of the polymer electrolyte as well as the catalytic activity. This results in the problem that the cell performance is gradually degraded during a long operation of the fuel cell. Additionally, in the case where a metal is used for the separator plate, metal ions eluted from the separator plate cause a further damage to the polymer electrolyte membrane and the catalyst layer.
Therefore, it is an object of the present invention to provide a method for effectively restoring the performance of a polymer electrolyte fuel cell In the case where the cell performance has been degraded owing to an accumulation of the impurities as described above.